The combustion temperature in a precombustion chamber is generally high and it is therefore effective in reducing NOx emissions to burn rich air-fuel mixture. Burning the rich mixture when the combustion temperature is high is effectively realized by the use of engines with a precombustion chamber. In the precombustion chamber-incorporated engines, to increase the combustion speed of the precombustion chamber almost to that of the direct injection type combustion chamber requires increasing the cross-sectional passage area of a communication hole connecting the precombustion chamber to the main combustion chamber. Increasing the cross-sectional area of the communication hole, however, lowers the ejection speed of the mixture flowing from the precombustion chamber into the main combustion chamber, resulting in a poor combustion in the main combustion chamber. Because the communication hole that connects the main combustion chamber and the precombustion chamber is located either at the central part of the cylinder or at one point of the cylinder periphery, the distance that the fuel mixture jet must travel is long, with the result that the fuel mixture and air in the main combustion chamber are not sufficiently mixed, producing HC emissions and smoke.
There has been an engine with a swirl chamber, developed to improve combustion. The engine with a swirl chamber includes a swirl chamber formed in the cylinder head or piston head, a communication hole communicating the swirl chamber to the main combustion chamber formed in the cylinder, and a fuel injection nozzle that sprays fuel into the swirl chamber. In this structure a swirl produced in the swirl chamber is mixed with a fuel injected into the chamber to form air-fuel mixture, which is subjected to a primary combustion. The flame and unburned mixture gas is elected from the swirl chamber through the communication hole into the main combustion chamber where the mixture is subjected to second combustion (Japan Patent Laid-Open No. 112613/1990).
The engine with a swirl chamber has a problem that unless the direction of swirl formed in the swirl chamber, the direction of sprayed fuel from the fuel injection nozzle and the fuel injection timing from the fuel injection nozzle match, the amount of emissions such as NOx and smoke will increase. Because in the swirl chamber-incorporated engine the communication hole connecting the precombustion chamber and the main combustion chamber is small, there is a throttling loss caused by the communication hole which reduces the engine output. The communication hole connecting the main combustion chamber and the precombustion chamber is generally located at the central part or outer peripheral part of the cylinder, so that the distance that the jet of mixture must travel is long, resulting in poor mixing of the fuel mixture with air in the main combustion chamber. This in turn produces emissions such as HC and smoke, and also reduces combustion speed and mileage.
Where the engine is constructed to have a precombustion chamber, increasing the combustion speed of the precombustion chamber almost to that of the direct injection type combustion chamber requires enlarging the cross sectional area of the communication hole, which connects the precombustion chamber and the main combustion chamber. When the cross-sectional area of the communication hole is increased, however, the speed of ejection from the precombustion chamber to the main combustion chamber is reduced, resulting in an insufficient combustion in the main combustion chamber.
To deal with this problem, a possible measure may involve installing the precombustion chamber at the center of the cylinder and providing two or more communication holes to connect the main combustion chamber and the precombustion chamber, both on the cylinder side. This increases the overall cross-sectional area of the communication holes and therefore lowers the throttling loss caused by the communication holes without reducing the energy of ejection from the precombustion chamber through the communication holes into the main combustion chamber.
Forming the precombustion chamber almost at the center of the cylinder head, however, raises a problem of reducing the space where the intake and exhaust ports are formed. This may be solved by forming the precombustion chamber in the piston in the precombustion chamber-incorporated engine. Considering the thermal energy dissipation from the precombustion chamber formed in the piston, the precombustion chamber may be constructed in a heat insulating structure.
In the engine with a precombustion chamber, the air flow in the precombustion chamber is active and the heat transfer rate between the hot burning gas and the inner wall of the precombustion chamber is high. Because flame and unburned fuel mixture gas is ejected at high speed from the precombustion chamber through the communication holes into the main combustion chamber, the heat transfer rate between the wall surface of the communication holes and hot burning gas passing through them is high.
There has been developed a heat insulating engine, which reduces heat dissipation from the combustion chamber to improve thermal efficiency. The heat insulating piston has its piston head formed in a heat insulating structure--which includes a reduced area of contact through a low thermal conduction Material, and an air layer--thereby keeping the temperature of the wall surface of the piston head high and that of the piston skirt low for sliding. The heat-resistant members forming the piston head are made of a low thermal expansion ceramics such as Si.sub.3 N.sub.4. Likewise, the material of the piston skirt uses a low thermal expansion metal such as low thermal expansion cast iron.
The heat insulating piston has a piston skirt mounted to the piston head. Although the metal of the piston skirt has a low thermal coefficient, the thermal expansion at high temperature is large when compared with that of the ceramics such as Si.sub.3 N.sub.4 that forms the piston head. Therefore, not only is the mounting structure between the piston head and the piston skirt is complicated but, when a heat insulating air layer is formed between the piston head and the piston skirt, the gas seal structure for high temperature and high pressure gas at the outer circumference of the piston becomes complex, increasing the manufacture cost. When the heat insulation air layer is not formed between the piston head and the piston skirt, the insulation level deteriorates, increasing the heat flow from the piston head to the piston skirt. This in turn increases heat dissipation from the combustion chamber, resulting in a degraded performance of the engine.
The ceramic piston disclosed in Japan Utility Model Laid-Open No. 93141/1987 is characterized in that the outer periphery of a ceramic crown with a cavity is fitted in the head portion of a cylindrical aluminum skirt, that the a plastic fluid material is installed in a gap between the outer periphery of the ceramic crown and the head portion, that the plastic fluid material is heated and pressurized to be deformed, joining the ceramic crown and the aluminum skit together, and that a cast iron ring is installed, together with the plastic fluid material, in the gap between the combustion chamber outer periphery and the head portion.
The engine with a precombustion chamber disclosed in Japan Patent Laid-Open No. 241049/1994 has a precombustion chamber formed in the piston head, a main combustion chamber formed in the cylinder, communication holes connecting the main combustion chamber and the precombustion chamber, and a fuel injection nozzle that injects fuel into the precombustion chamber. As the piston nears the top dead center, the fuel injection nozzle protrudes into the nozzle insertion hole formed in the piston head. The communication holes are formed inclined with respect to the cylinder axis in the circumferential direction so that the direction of inflow into the precombustion chamber is reverse to the direction of swirl in the main combustion chamber and that the direction of ejection from the precombustion chamber to the main combustion chamber is in the same direction as the swirl in the main combustion chamber.
Japan Patent Laid-Open No. 149725/1990 discloses a direct injection type alcohol engine. In this alcohol engine, an injector injects alcohol fuel into a combustion chamber comprising a recessed portion formed in the top of the piston. The top clearance when the piston is at the top dead center is set to 2-5 mm. The combustion chamber is formed with a lip at the opening edge, and the circumferential wall almost perpendicular to the direction in which the fuel is sprayed from the injector is formed along the circumferential direction in the combustion chamber.
Japan Utility Model Laid-Open No. 100528/1985 discloses a piston for direct injection type diesel engines. This piston is characterized in that the opening portion of the combustion chamber formed in the piston top portion is provided with a closing member, that fuel injection passages communicating the combustion chamber and the cylinder chamber are formed in the closing member in such a way that they are oriented in the direction that produces a swirl in the combustion chamber and extend almost parallel to the direction in which the fuel is ejected from the fuel injection nozzle. This piston uses a heat-resistant material in the top portion of the combustion chamber subjected to severe thermal load to secure a sufficient durability, but it does not employ a heat insulating structure for the precombustion chamber.
Japan Utility Model Laid-Open No. 141446/1985 discloses a ceramics-incorporated piston. The ceramics-incorporated piston has an assembly which consists of a ceramic member and a heat- and corrosion-resistant metal ring joined to the outer periphery of the ceramic member. The assembly is threaded into the piston body of aluminum alloy to install the ceramic member in the recess of the piston body on the combustion chamber side. The combustion chamber is therefore formed of a ceramic member, which is threaded into the piston body of aluminum alloy.
Japan Patent Laid-Open No. 109267/1988 discloses a piston for internal combustion engines. This piston has a piston body and a ceramic cavity member installed in the top part of the piston body by casting. That is, the piston body is cast around the cavity member, with a heat insulating layer--which is a molded ceramic fiber attached with an inorganic material--covering the outer surface of the cavity member. The heat insulating layer of ceramic fiber is provided around the combustion chamber to prevent ingress of molten metal of the piston body into the combustion chamber and to secure the strength of the combustion chamber. The piston, however, does not provide a structure for joining the heat-resistant metal and the heat insulating layer.
In the piston with a combustion chamber, it is preferred that, when a combustion chamber structure that forms the combustion chamber is installed in the cavity formed in the piston body, the combustion chamber structure be made of a heat-resistant, high temperature high strength material if the piston body is made of aluminum alloy or cast iron. In that case, one problem that should be addressed is that the combustion chamber structure should be firmly secured to the piston body while minimizing the dissipation of heat energy from the combustion chamber formed in the combustion chamber structure.